Current treatment for End Stage Renal Disease (ESRD) essentially consists of a hemodialysis process, wherein blood to be cleaned flows on one side of a semipermeable membrane and a physiologic solution (e.g., a dialysate) flows on the other side of the membrane, whereby toxins in the blood are transferred from one side to the other. The primary driving force in this treatment is diffusion. This process is generally effective in removing small Molecular Weight (MW) toxins such as urea and creatinine. However, this process is much less effective in removing middle range MW substances, e.g., substances having a molecular weight higher than about 1 kDa, because of a low diffusion coefficient of such substances.
To a much lesser extent hemofiltration is used as a treatment modality. As in hemodialysis, the blood flows on one side of the semipermeable membrane, however, there is no dialysate flow on the other side. Instead a pressure gradient is established across the membrane so that a portion of the blood plasma water is filtered across. With the plasma water, toxins are convectively removed from the blood. Sterile non-pyrogenic replacement fluid is added to the blood either prior to or after it enters a hemofilter. The replacement fluid replaces the plasma water which is filtered across the semi-permeable membrane during the hemofiltration process. This process is generally less efficient at removing the small MW toxins compared to dialysis, but more efficient at removing the middle MW substances.
Hemodiafiltration combines dialysis and hemofiltration. Dialysate fluid flows on the other side of the semi-permeable membrane resulting in diffusion of toxins. At the same time, a pressure gradient across the membrane is maintained resulting in a high filtration rate. As with hemofiltration, sterile non-pyrogenic replacement fluid is added to the blood either prior to or after it enters a hemodiafiltration cartridge. As a result of this combination, hemodiafiltration is efficient at removing small molecules, e.g., creatinine and urea, by diffusion as well as removing large quantities of middle range MW substances, by convection.
State of the art designs for hemodiafiltration filters are substantially equivalent to those of high flux dialyzers. Such filters consist of a bundle of hollow fibers in a cylindrical housing. During operation of the hemodiafiltration system, replacement fluid is injected into the blood either upstream (pre-dilution) or downstream (post-dilution) of the high flux dialyzer.
Diafiltration devices using pre-dilution or post-dilution schemes have inherent efficiency limitations. Pre-dilution schemes allow for relatively unlimited filtration, however, because the blood is diluted prior to reaching the filter, the overall mass transfer of small solutes by diffusion is decreased. Post-dilution schemes have the advantage of keeping blood toxin concentrations high, resulting in more efficient diffusion and convection of solutes, however, the increased concentration of blood cells and the resultant higher blood viscosity during filtration, poses a limit on the amount of plasma water that can be filtered. Even the existing multistage paired filtration dialysis system disclosed in U.S. Pat. No. 5,194,157 faces the same filtration limitations due to its design.